The present invention relates to an apparatus and method for early arc fault detection and more particularly relates to an apparatus and method for both a stand alone early arc fault detector and an early arc fault detector combined with a circuit interrupter device.
Circuit breakers, fuses and ground fault circuit interrupters (GFCIs) are commonly used devices for protecting people and property from dangerous electrical faults. Fatalities and loss of property, however, still occur, being caused by electrical faults that go undetected by these protective devices. One such type of electrical fault that typically goes undetected are arc faults. Arcs are potentially dangerous due to the high temperatures contained within them. Thus, they have a high potential of creating damage, mostly through the initiation of fires. An arc, however, will only trip a GFCI it if produces sufficient current leakage to ground. In addition, an arc will trip a breaker only if the current, flowing through the arc, exceeds the trip parameters of the thermal/magnetic mechanism of the breaker. Therefore, an additional type of protection device is needed to detect and interrupt arcs that do not fit these criteria. An arc detector whose output is used to trigger a circuit interrupting mechanism is referred to as an arc fault circuit interrupter (AFCI).
According to the Consumer Product Safety Commission (CPSC) in 1992, it was estimated that xe2x80x9cthere were 41,000 fires involving home electrical wiring systems . . . which resulted in 320 deaths, 1600 injuries and $511 million in property losses.xe2x80x9d The CPSC further stated that xe2x80x9can electrically caused fire may occur if electrical energy is unintentionally converted to thermal energy and if the heat so generated is transferred to a combustible material at such a rate and for such a time as to cause the material to reach its ignition temperature.xe2x80x9d The two main causes of unintentional conversion of electrical energy to heat are excessive current and arcing. Circuit breakers and fuses are currently available to mitigate the results of excessive current, but no commercial system exists to mitigate arcing.
A dangerous condition may develop whenever prolonged arcing exists regardless of whether it involves industrial, commercial or residential power lines. However, mobile homes and especially homes with antiquated wiring systems are particularly vulnerable to fires started due to electrical causes. CPSC studies have shown that the frequency of wiring system fires is disproportionately high in homes over 40 years old.
The causes of arcing are numerous, for example: aged or worn insulation and wiring; mechanical and electrical stress caused by overuse, over currents or lightning strikes; loose connections; and excessive mechanical damage to insulation and wires. Two types of arcing occur in residential and commercial buildings: contact arcing and line arcing. Contact (or series) arcing occurs between two contacts in series with a load. Therefore, the load controls the current flowing in the arc. Line (or parallel) arcing occurs between lines or from a line to ground. Thus, the arc is in parallel with any load present and the source impedance provides the only limit to the current flowing in the arc. It is important for any arc detection system to be able to detect both contact and line arcing and to act appropriately depending upon the severity of the arc.
An example of contact arcing is illustrated in FIG. 1. The conductors 114, 116 comprising the cable 110, are separated and surrounded by an insulator 112. A portion of the conductor 114 is broken, creating a series gap 118 in conductor 114. Under certain conditions, arcing will occur across this gap, producing a large amount of localized heat. The heat generated by the arcing might be sufficient to break down and carbonize the insulation close to the arc 119. If the arc is allowed to continue, enough heat will be generated to start a fire.
A schematic diagram illustrating an example of line arcing is shown in FIG. 2. Cable 120 comprises electrical conductors 124, 126 covered by outer insulation 122 and separated by inner insulation 128. Deterioration or damage to the inner insulation at 121 may cause line fault arcing 123 to occur between the two conductors 124, 126. The inner insulation could have been carbonized by an earlier lightning strike to the wiring system, or it could have been cut by mechanical action such as a metal chair leg cutting into an extension cord.
The potentially devastating results of arcing are widely known and a number of methods of detecting arcs have been developed in the prior art. A large percentage of the prior art refers to detecting the high frequency signals generated on the AC line by arcs. FIG. 3 shows the wide spectrum noise 162 produced on the AC line by an arc. It is superimposed over the AC line voltage 164. An analysis of the arc waveform, using a frequency spectrum analyzer, shows that the overtones and high frequency harmonics contained within the waveform extend well into the GHz range. A graph illustrating the frequency spectrum analysis of the waveform 162 shown in FIG. 3 is shown in FIG. 4.
One major problem associated with any type of arc detection is false tripping. False tripping occurs when an arc detector produces a warning output, or disconnects a section of wiring from the voltage source, when a dangerous arcing condition does not actually exist. The two major causes of false tripping are normal appliance arcing and the inrush currents created by inductive and capacitive appliances. These two situations generate high frequency signals on the power line that are very similar to those generated by dangerous arcing. Thus, to be viable commercial devices, arc detectors must be able to distinguish arcing signals from the signals created by normal appliance use.
A wide range of prior art exists in the field of arc detection. Some of the prior art refers to specialized instances of arcing. For example, U.S. Pat. No. 4,376,243, issued to Renn, et al., teaches a device that operates with DC current. U.S. Pat. No. 4,658,322, issued to Rivera, teaches a device that detects arcing within an enclosed unit of electrical equipment. U.S. Pat. No. 4,878,144, issued to Nebon, teaches a device that detects the light produced by an arc between the contacts of a circuit breaker.
In addition, there are several patents that refer to detecting arcs on AC power lines that disclose various methods of detecting high frequency arcing signals. For example, U.S. Pat. Nos. 5,185,684 and 5,206,596, both issued to Beihoff et al., employ a complex detection means that separately detects the electric field and the magnetic field produced around a wire. U.S. Pat. No. 5,590,012, issued to Dollar, teaches measuring the high frequency current in a shunted path around an inductor placed in the line, which can be the magnetic trip mechanism of a breaker. In a second detection circuit, proposed by Dollar, high frequency voltage signal is extracted from the line via a high pass filter placed in parallel with any load.
Various methods can be found in the prior art to authenticate arcing and to differentiate arcing from other sources of noise. Much of the prior art involves complicated signal processing and analysis. U.S. Pat. No. 5,280,404, issued to Ragsdale, teaches looking for series arcing by converting the arcing signals to pulses and counting the pulses.
In addition, several patents detect arcing by taking the first derivative or second derivative of the detected signal. For example, U.S. Pat. No. 5,224,006, issued to MacKenzie et al., and U.S. Pat. Nos. 5,185,684 and 5,206,596, issued to Beihoff et al, disclose such a device.
Blades uses several methods to detect arcs as disclosed in U.S. Pat. Nos. 5,223,795, 5,432,455 and 5,434,509. The Blades device is based on that fact that detected high frequency noise must include gaps at each zero crossing, i.e., half cycle, of the AC line. To differentiate arcing from other sources of noise, the Blades device measures the randomness and/or wide bandwidth characteristics of the detected high frequency signal. The device taught by U.S. Pat. No. 5,434,509 uses the fast rising edges of arc signals as a detection criterion and detects the short high frequency bursts associated with intermittent arcs.
U.S. Pat. No. 5,561,505, issued to Zuercher et al., discloses a method of detecting arcing by sensing cycle to cycle changes in the AC line current. Differences in samples taken at the same point in the AC cycle are then processed to determine whether arcing is occurring.
The arc detector of the present invention functions to monitor and sense the line voltage and current present on the AC power line for the occurrence of arcing. Both high frequency energy and AC line frequency energy are utilized in the detection of arc faults. The output of the detector can be used to activate a circuit interrupting mechanism, sound an audio alarm and/or alert a central monitoring station.
The present invention utilizes a single transformer as a grounded neutral fault detector and as an arc fault detector. This is accomplished by splitting one of the wires passing through the ground/neutral transformer of the GFCI into parallel paths such that a small portion of the current in the wire flows around the transformer. A differential current is then produced in the coil that is proportional to the current flowing through the AC line. The differential current is picked up by the ground/neutral transformer and is available for further processing. Thus, the ground/neutral transformer can be utilized as part of the arc detection circuitry. The differential current generated in the ground/neutral coil is relatively small, thus the ground fault and ground/neutral fault detection capabilities of the GFCI are unaffected. The use of the same coil for two purposes provides tight integration between the GFCI and the AFCI circuits.
The present invention also includes a novel early arc detection circuit capable of detecting high impedance on the AC line. Conventional line testers typically load the AC line in order to detect high impedance on the AC line at a particular moment in time. The present invention uses a hardwired test circuit, which periodically tests the AC line with a controlled load. The circuit ensures that the line is loaded for only a short period when the AC line voltage is low, thus keeping power dissipation in the device low. Using this method, points of high impedance on the line, which may lead to arcing and dangerous heating, can be detected before dangerous arcing occurs.
The early arc detection circuit functions to generate pulses proportional to the line impedance to determine the impedance of the AC line. An inductor/capacitor network is utilized to generate pulses proportional in size to the drop in line voltage caused by the introduction of a controlled load. The drop in voltage is itself proportional to the line impedance between the AFCI/GFCI and the AC voltage source.
The arc detector of the present invention can be implemented as a standalone device or can be implemented in combination with an existing circuit interrupting device. The term xe2x80x98circuit interrupting devicexe2x80x99 is defined to mean any electrical device used to interrupt current flow to a load and includes, but is not limited to devices such as Ground Fault Circuit Interrupters (GFCIs), Immersion Detection Circuit Interrupters (IDCIs) or Appliance Leakage Circuit Interrupters (ALCIs).
A main feature of the arc detector of the present invention is that it combines an arc detector, i.e., arc fault circuit interrupter (AFCI) with other types of circuit interrupting devices such as a GFCI to create an AFCI/GFCI multipurpose device. In the case of a GFCI, the arc detection circuitry can be placed onboard the same silicon chip typically used in today""s GFCI devices. Indeed, some of the pins of commonly used GFCI integrated circuits can be converted for multifunction operation. The AFCI can be powered from the same power supply that provides power to the circuit interrupting device. This combined approach results in reduced manufacturing costs. The mechanical parts of the circuit interrupting device such as the trip relay and the mechanical contact closure mechanisms now serve dual purposes. In addition, adding AFCI circuitry to an existing circuit interrupting device is a logical enhancement of such present day devices. In particular, it is logical to enhance a GFCI with AFCI circuitry since a GFCI can detect arcing in certain situations including any condition whereby an arc produces leakage current to ground.
In the AFCI/GFCI device of the present invention, the current waveform present on the AC line is extracted via a toroidal current to voltage transformer that is shared with the circuit interrupting device. An impedance splits the current flow into two portions so as to generate differential current proportional to the current flowing through the line conductors. The voltage that is generated across the windings of the transformer is fed to two separate paths.
In the first path, the 50 or 60 Hz AC line frequency content of the transformer output is filtered from the input signal. This AC line frequency signal provides an indication of the amount of current flowing through the AC power line. In the second path, the high frequency content of the transformer output is filtered from the input signal. The high frequency signal is indicative of the level of arcing present on the AC power line.
Within each of the two paths, the signals are filtered by a second stage filter and then rectified. The two rectified signals are each split to produce peak and average levels for the AC line frequency and high frequency signals. Excessively high peaks in either the AC line frequency or high frequency path instantly causes the relay mechanism of the AFCI/GFCI to trip, disconnecting the load from the power source.
The absolute average levels of the AC line frequency and high frequency signals are converted to a DC potential and compared to a set of predefined voltages. If the average high frequency signal is greater than the level expected from normal appliance arcing at the associated average AC line frequency level, then an output signal is generated. This output signal is then used to trip the device or produce an alarm both being controlled via a timer mechanism. A user can disable the AFCI function temporarily or permanently so that devices with normally high levels of arcing, such as arc welders, can be operated without tripping the arc detector.
The detection of high average AC line frequency or high frequency signals causes the device to immediately trip. This immediate tripping cannot be disabled via the timer mechanism described above. In addition, the ground fault protection mechanism and excessive arc current and AC line current detection cannot be disabled. This is so a user is continually protected from the potential dangers associated with these conditions.
An advantage of the present invention is that separating the detection of the AC line current and the high frequency energy generated by the arc provides increased immunity to noise. The arc detection device detects the current flowing in the AC line across a wide range of frequencies. By splitting the two current signal components and setting a maximum permitted level of high frequency component for a given level of AC line current, the arc detector provides increased immunity to noise.
In addition, the arc detector of the present invention simultaneously performs average and peak detection of AC line current and high frequency arcing signal. The peak AC line current and high frequency arcing signals are detected to provide an immediate response to large increases in either arcing or AC line current. The arc detector will trip the relay the instant either the peak AC line current signal or the peak high frequency arcing signal crosses a predetermined threshold.
The arc detector also incorporates a fast trip circuit which functions to open the relay when excessive average AC line current and high frequency arcing levels are detected. If either the average AC line current or the average high frequency arcing signal rises above a level considered to be dangerous, the device will trip very quickly. The maximum level permitted for the average AC line current is approximately 1.5 times the rated AC line current. The limit set for the average high frequency signal is a level of average arcing that is known to be dangerous.
When the levels of average AC line current and high frequency signal are lower than their respective maximums, the arc detector utilizes various trip levels for arcing, dependent upon the level of the average AC current flowing. Furthermore, the arc detector trips at a slower speed at these lower and thus less dangerous arcing levels. This slower trip response time provides noise immunity against short lived noise and arcs, such as arcs generated when toggling a switch. By incorporating various trip times, dependent on the level of arcing detected, the arc detector can extinguish dangerous arcs quickly while providing high noise immunity for lower level arcs.
The arc detector also incorporates an automatic bypass timer to permit otherwise normally safe arcing. Rather than include an on/off fixed switch, which would function to completely enable or disable the arc detector, the present invention incorporates a logical switch. This logic switch provides a user with the option of disabling the arc detector for as long as the switch is off or disabling the arc detector temporarily while arcing appliances are in use. This permits the use of appliances that normally generate high amounts of arcing that would otherwise cause the arc detector to trip. When the arc detector is temporarily disabled, it automatically returns to the enabled state after the appliance has been disconnected. This scheme has the advantage that the device cannot accidentally be permanently disabled by the user. An important feature of this scheme is that the arcing appliance can be turned on and off within the given time period without tripping the arc detector.
Further, the arc detector includes circuitry to transmit messages on the AC power line using any power line carrier system pinpointing the location of arc fault. The present invention incorporates a communications circuit, which utilizes a power line carrier signal such as generated by the CCS product line manufactured by Leviton Manufacturing, Little Neck, N.Y. Using well known power line carrier techniques the arc detector can communicate with other devices such as a monitoring station. Each arc detector would have a unique address. A relationship is then established between the address assigned to the arc detector and its location. When an arc fault is detected, a signal is sent over the power lines to a monitoring station which alerts personnel of not only the occurrence of the arc fault but also its location. This is helpful especially if the AFCI/GFCI device is installed in a remote location. This feature has applicability in industrial and commercial locations where central arc fault supervision over a complex AC electrical wiring system is needed.
Today, AC power lines are not only used for supplying AC line current but they are also used as a media for communications as in Leviton Manufacturing""s CCS line of power line carrier devices, CEBus compatible devices, LonWorks compatible devices, power line carrier based intercoms, TV signal transmission/reception equipment, telephone communication devices, etc. The arc detector of the present invention incorporates a filter circuit which permits the detection of arc faults while communications over the AC power lines is occurring. The filter circuit functions to remove frequencies below 500 KHz. On the other end of the frequency spectrum, although arcing generates frequencies into the GHz range, for simplicity, efficiency and reduced cost the arc detector of the present invention limits detection of high frequency signals to approximately 20 MHz.